1. Field of the Invention
The present invention relates to downhole well tools. In particular, the invention relates to an articulated joint between adjacent, operatively connected tubular sections of and elongated instrument housing.
2. Description of the Prior Art
For many reasons, a well bore may follow a tortured course having one or more turns; some of relatively short radius. Standard drill pipe length is about thirty feet. Notwithstanding the apparent strength and rigidity of drill pipe, a thirty foot length is capable of considerable flexure. For such reason, a traditional drill string may accurately be perceived as a flexible drive shaft capable of rotation about the longitudinal pipe axis over a relatively small radius of arc. Downhole drill motors supported by coiled tubing are capable of boring even smaller radius arcs.
Generally, downhole well tools are lowered along the inner bore of casing, drill pipe or tubing within a well bore. Consequently, the downhole tool substantially follows the same undulations as the drill string or tubing. However, tool housings, especially electronic measuring or control instruments are not constructed of the same materials as drill string and cannot accommodate the same degree of bending. Nevertheless, some downhole tools such as Measuring While Drilling (MWD) systems or steering tools require substantial total tube length to accommodate the necessary component volume within a relatively small inside diameter. Consequently, the tubular housings for such instruments must be segmented into two or more length sections. Since the two or more length sections are functionally one tool, the several tubular housing sections must communicate to function as a unit. At the same time, the several sections must maintain a relatively consistent angularity about the longitudinal axis between the leading or lower end of the tool and the trailing or upper end of the tool.
U.S. Pat. No. 4,842,059 titled: FLEX JOINT INCORPORATING ENCLOSED CONDUCTORS partially addresses these issues with a double ball-and-socket style of universal joint. To transfer torque about the longitudinal axis of a multiple tube instrument, ball-and-socket joints between the tubes are pinned to prevent relative axial rotation between a ball element and a socket element. Dynamic pressure seals between the ball and the respective socket permits a positive pressure fluid chamber between cable connector plugs respective to each of the two instrument length sections. The positive pressure chamber objective of the ""059 disclosure is to protect the electrical continuity and electrically isolate the several signal carrier conduits passing between adjacent instrument section. A spring loaded annular piston maintains the chamber pressure to exclude unwanted fluids.
U.S. Pat. No. 5,836,388 titled FLEXIBLE JOINT FOR DOWNHOLE TOOL and U.S. Pat. No 5,769,558 titled FLEX JOINT both provide sealed, flexible joints between adjacent MWD tool sections. The structural link between adjacent tool sections comprises a pair of wound coil springs encased in an integral rubber boot. The injection molded rubber boot provides electrical insulation and environmental isolation from the borehole. Although the coil springs are capable of transmitting torque from one tool section to the other, the torque is transmitted through a substantial angular displacement. Additionally, the springs permit considerable elongation and contraction between the adjacent tool ends. Moreover, considerable force is required to bend the boot encased spring.
It is an objective of the present invention to provide a flexible joint between adjacent downhole instrument housings that will neither elongate nor permit significant angular displacement between adjacent housing tubes.
Another object of the present invention is a flexible joint between adjacent downhole instrument housings that will protect the communication continuity of signal carriers between the adjacent housings.
Also an object of the present invention is a flexible joint between adjacent downhole instrument housings that is inexpensive to fabricate, assemble, service and repair.
A further object of the present invention is a flexible joint between adjacent downhole instrument housings having no need for a pressure compensation system to protect the insular environment around the signal carriers between the housings.
Broadly, the present invention comprises a flexible, fluid impermeable sheath for enclosing signal carrying conduit that is threaded through a torque transmitting universal joint. The universal joint mechanically links two adjacent housings of an articulated instrument. The housings are long tubes for encapsulating electronic components and circuitry. Two embodiments of the invention provide an enclosed passageway between the adjacent housings for threading the signal carriers. The passageway comprises a flexible wall tube having considerable radial strength such as a bellows or hydraulic fluid power conduit. A third invention embodiment encapsulates the carrier conduits with an elastomer that is molded within a relatively thin, fluid impermeable sheath The sheath has a fluid tight connection at opposite ends to respective housings.
The mechanical joint of the present invention comprises a Cardan type of universal joint wherein the meshed joint fingers of two joint bases are pivotally connected by an open ring spyder. Four spindles projecting in a common plane radially from the outer periphery of the ring pivotally secure each of the four meshed fingers. An open center area of the ring accommodates through passage of a flexible, substantially fluid impermeable signal carrier sheath between adjacently joined ends of the instrument housings.
In one embodiment of the invention, the flexible sheath may take the form of a flexible, high pressure hose of the type commonly used for high pressure hydraulic systems. Hose for this purpose may be constructed with tubular walls that are reinforced with braided or woven steel wire. Opposite ends of the hose may be secured to respective ends of the adjacent instrument housings by traditional tubing nuts for a pressure tight connection around an aperture through the respective housing end walls. The hose is threaded through the open center of the universal joint spyder ring and the signal conduit are threaded through the open hose channel.
In another embodiment of the invention, the sheath comprises a cylindrical bellows having a high pressure mechanical attachment at opposite ends of the sheath to respective bore plugs. The bore plugs seal apertures through the respective housing ends for physical passage of the signal carrying conduits which may take the form of electrical wiring, optical communication fibers or fluid conduits. The signal carriers are the operationally unifying arteries between instrument components that are physically located within the spacial volumes enclosed by the tubular walls of the respective instrument housings. The signal carriers are threaded through an open passageway within the bellows. The bellows convolutions provide sufficient structural integrity to oppose a pressure collapse or penetration at low to moderate well depths and pressures. Hence, the assembly pressure within the bellows sheath is atmospheric and no downhole pressure compensation system is required.
Another embodiment of the invention, especially suitable for extremely high pressure, deep well applications, provides a flexible, fluid impermeable sheath for enclosing the signal carriers. In this embodiment, the sheath is also secured to the housing end walls with a fluid tight connection around an end wall aperture. However, the sheath also confines a substantially solid filler of flexible elastomer material such as silicone rubber that is injected into the sheath after the signal carriers are threaded through the sheath. This elastomer encases the signal carriers within the outer sheath.
A bore plug may be provided within each of the adjacent instrument housings inside of the first or outer bore plug. Linking signal carriers are connected at respective inner ends to a bulkhead gang-connector mounted within the interior plug and to a gang-connector mounted in the outer plug.
Preferably, the outer bore plugs are secured to opposite ends of the flexible sheath that joins them as a singular unit. Additionally, the outer plugs are conveniently removable from the housing end bores to facilitate separation and disconnection of the singular unit from either or both of the housings.
The universal joint of the present invention requires little force to deflect since the flexing structure carries no load except the borehole pressure. Additionally, the invention provides azimuth alignment between the top and the bottom modules and prevents relative rotation or axial displacement about the (Z) axis. Since the universal joint of the present invention does not require a separate pressure compensation section, the joint may be made with minimum length. Cardan universal joints require little force to deflect since the flexible element in the joint carries no external pressure load except for the borehole pressure.